This invention relates generally to magnetometers and more particularly to the calibration of three axis magnetometers.
Three axis magnetometers, that is magnetometer devices having three mutually orthogonal magnetic field component sensing elements, are used in a variety of applications. For example, such a magnetometer can be used to determine magnetic characteristics or effects of magnetic minesweeping apparatus or to determine magnetic signatures of ships and other vessels or equipment. Magnetometers of the three axis type are also useful in a variety of other applications well known to those skilled in the art to which the invention pertains. Much attention has been paid to the problems of adjusting or correcting the magnetic sensors to overcome errors arising from a variety of factors that are difficult to control or predict in the manufacture and assembly of such devices. Adjustments, corrections and calibration of a magnetometer are typically carried out in an artificially controlled field situation generated by a Helmholtz room. The Helmholtz room comprises an enclosure defined by a plurality of coils arranged in mutually prependicular planes and energizeable to produce magnetic fields the components of which are capable of being rather accurately calculated. When the typical magnetometer has been adjusted and calibrated, it is then removed from the Helmholtz room and placed into service. The magnetometer generally is required to be carefully positioned, with its axes of sensitivity in known directions relative to the earth's magnetic field and, for certain applications, the magnetometer device is disposed in a location that is relatively inaccessible for recalibration, for example in an underwater test site from which readings are telemetered by cable to a remote control and data instrumentation station.
Now, magnetometers, by their nature, tend to be delicate and subject to changes in response and sensitivity due to handling, adverse magnetic conditions experienced between calibration and the time of use, and the like. Accordingly, whenever magnetometer data or readings are obtained during use that are not entirely consistent with those expected or predicted, the calibration and adjustment of the magnetometer itself becomes suspect. Heretofore, it has been necessary in such circumstances to retrieve the magnetometer, recalibrate and adjust it in a Helmholtz enclosure, replace it at the test site, and obtain new data. Of course such a procedure is time consuming, costly, and the validity of the data is still open to question in that the magnetometer may have suffered a duplicate loss of calibration.